feat(neuron): emit response.in_progress between created and output_item.added

Refs #7.

OpenAI's Responses API spec emits `response.in_progress` between
`response.created` and the first output-item event to mark
"request validated, model is generating". Some Responses-API
clients distinguish loading-spinner vs streaming-spinner UI based
on which event arrived last; emitting both keeps the wire shape
matched.

Carries the same shell as `response.created` (status=in_progress,
empty output, no usage yet) — both events are payload-light
bookkeeping, distinguished only by the event name.

The hosted-tool event families remaining in #7 (web_search_call,
code_interpreter_call, file_search_call, image_generation_call)
stay deferred until the underlying tools exist in neuron.

Updated `full_stream_emits_expected_event_sequence` to assert the
new event lands in position 1; downstream indexing shifted by one
across the existing test assertions. CI green, fmt + clippy clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/cortex-core/src/responses.rs

@@ -212,6 +212,11 @@ pub struct ResponsesUsage {
 /// Responses API.
 pub mod events {
     pub const CREATED: &str = "response.created";
+    /// Fired between `response.created` and the first output-item
+    /// event. Marks "request validated, model is generating" —
+    /// some clients use it to differentiate the "warming up" state
+    /// from "streaming tokens" in their UI.
+    pub const IN_PROGRESS: &str = "response.in_progress";
     pub const OUTPUT_ITEM_ADDED: &str = "response.output_item.added";
     pub const CONTENT_PART_ADDED: &str = "response.content_part.added";
     pub const OUTPUT_TEXT_DELTA: &str = "response.output_text.delta";